The present invention relates to an electric motor with active hysteresis-based control of winding currents, and/or having an efficient stator winding arrangement, and/or adjustable air gap. The present invention also relates to the stator, windings and air gap adjustment mechanism included in the electric motor, as well as a method and system for controlling the torque produced by the motor using active hysteresis-based control of the motor""s winding currents. The motor and its associated components are particularly well-suited for use in an electrically powered vehicle, as well as in hybrid vehicles using both electric power from a battery and electric power derived from a fuel-burning engine.
In the field of electrically-powered vehicles and hybrid vehicles, it is known that efficiency and versatility of a motor are key elements in a successful design. Another key element is the vehicle""s weight and the motor""s contribution to weight. These key elements have a significant impact on the range of the vehicle and its usefulness to consumers. These key elements also affect one another. A reduction in weight, for example, may have a negative impact on versatility. In this regard, removal of a gearing mechanism to reduce weight may limit the speed of the vehicle and hence its versatility, while improving the vehicle""s efficiency and overall range.
Not all improvements, however, have negative implications with regard to other key elements. An improvement in one key element actually may provide a synergistic improvement in another of the key elements. Versatility, for example, can reduce the overall weight of a vehicle. Motors having a wide range of torque-producing speeds are more versatile and also result in a reduced need for heavy gearing elements and complicated or electrically inefficient controlling arrangements. Thus, an improvement in the motor""s versatility translates into a reduction in the vehicle""s overall weight and improved efficiency. The present invention aims to maximize the positive interplay between these key elements, while at the same time reducing the negative impact between the key elements.
There are many commercially available electric motors, some of which may provide improvements in one or more of the key elements described above.
A primary object of the present invention is to overcome the disadvantages associated with known electric motors, by maximizing the synergistic interplay between the aforementioned key elements.
Another object of the present invention is to provide an electric motor having a controller capable of performing active hysteresis-based control of winding currents in a manner dependent on the desired torque or winding current level, the speed of the motor (RPM), and/or other variables, for example, to selectively balance a switching efficiency of the motor""s controller and resistive losses in the windings and/or to selectively provide a reduction in motor noise.
Yet another object of the present invention is to provide a stator having an electrically and magnetically efficient winding arrangement.
Still another object of the present invention is to provide a mechanism capable of adjusting the motor""s air gap, thereby improving the versatility of the motor in a gear-less manner by allowing the motor to produce a high level of torque at low speeds, while the air gap is small, and allowing the motor to continue producing torque, when the air gap is larger, at higher speeds than would be permitted with the smaller air gap.
Yet another object of the present invention is to provide a discharge circuit for automatically discharging a voltage on a power bus between a motor""s controller and the motor itself after the voltage on the power bus decays to a predetermined decay minimum.
To achieve these and other objects, the present invention provides a stator for an axial flux motor powered using at least two phases of electrical current. The stator includes a series of stator windings. The series is arranged in a circle which defines a circumference of the stator. The series of windings have arc sections, each of the arc sections containing windings associated with only one of the phases. The arc sections are arranged such that, when current flows through the windings, each arc section provides a magnetic field which, at any given instant of time during current flow, alternates in direction along a length of each arc segment.
The windings in each arc section may be arranged so that no two arc segments carry current of identical phase. Alternatively, more than one arc section may be associated with each phase. Preferably, the distribution of arc sections provides a favorable symmetry around the stator""s circumference. For example, in a three phase, six arc section motor, the two arc sections associated with each phase are 180 degrees apart, which provides symmetry with respect to the spin axis. Other symmetries may be achieved using different configurations.
The stator preferably includes an annular stator core. The annular stator core includes a radially inner surface, a radially outer circumferential surface, and two sides extending therebetween. The annular stator core further includes slots which extend between the radially outer circumferential surface and the radially inner surface to define teeth on one of the two sides of the annular stator core. The stator windings are defined by conductive material extending through the slots and around the teeth.
The stator windings may be defined by serpentine-shaped conductors which fit between and around the stator""s teeth, each serpentine-shaped conductor being coextensive with a respective one of the arc segments.
The serpentine-shaped conductors in each arc segment are stacked upon one another with an electrically insulative material disposed therebetween. The stack in each arc segment preferably includes a first set of substantially identical serpentine-shaped conductors stacked alternatingly through the stack, and a second set of serpentine-shaped conductors sandwiched between the serpentine-shaped conductors of the first set. The serpentine-shaped conductors of the second set are substantially identical to one another but different from those of the first set in that, when the first and second sets are alternatingly stacked, portions of the serpentine-shaped conductors in the first set which are outside of the slots extend around opposites sides of the teeth from portions of the serpentine-shaped conductors of the second set which are also outside of the slots.
The electrically insulative material between the conductors preferably includes openings through which adjacent ones of the serpentine-shaped conductors in each stack are electrically connected. The openings are arranged so that current flowing through each arc segment flows end-to-end through each serpentine-shaped conductor of that arc segment.
In order to reduce resistive losses in the portions of the serpentine-shaped conductors which are outside of the slots, such portions may have a larger cross sectional area than other portions of the serpentine-shaped conductors which are located in the slots.
The foregoing stator is preferably incorporated into an axial flux motor powered using at least two phases of electrical current. In addition to the stator, the axial flux motor includes a rotor having an outer circumference carrying magnetic field producing elements. The magnetic field producing elements are arranged so as to produce a magnetic field that alternates in direction around the circumference of the rotor. Preferably, the magnetic field producing elements are permanent magnets.
The stator is axially spaced from the rotor to define an air gap therebetween. In addition, the stator is selectively movable in an axial direction from the rotor to vary this air gap. Preferably, a mechanism is provided for axially moving the stator and thereby varying the air gap.
The motor can be arranged in a dual stator configuration. In this regard, a second stator may be provided in accordance with the present invention on an opposite side of the rotor from the first stator. The second stator preferably is arranged so as to constitute a mirror image of the first stator.
Preferably, both the first and second stators are axially spaced from the rotor to define first and second air gaps, and both stators are selectively movable in the axial direction to vary the air gaps. First and second mechanisms for varying the air gaps may be provided.
A coupling mechanism also may be provided for coupling actuation of the first mechanism to actuation of the second mechanism, so that variations in the first air gap produce substantially identical variations in the second air gap.
The present invention also provides a winding arrangement for a motor. The winding arrangement includes a magnetic core having teeth defined by slots in the magnetic core, and at least one conductive winding passing in and out of the slots. The conductive winding has portions outside of the slots which have a larger cross sectional area than portions of the conductive winding which are located inside the slots.
Preferably, the winding arrangement includes serpentine-shaped conductors which fit between and around the teeth.
The present invention also provides a dual stator motor with selectively variable air gaps. The dual stator motor includes first and second stators which are axially movable, a rotor rotatably mounted between the first and second stators for rotation in response to magnetic fields generated by the stators, a first mechanism for moving the first stator axially with respect to the rotor to vary a first air gap, and a second mechanism for moving the second stator axially with respect to the rotor to vary a second air gap.
A coupling mechanism preferably is provided for coupling actuation of the first mechanism to actuation of the second mechanism so that the first air gap remains substantially the same as the second air gap.
Each of the first and second mechanisms preferably includes a cam hub having camming members, and stator camming elements mounted to a respective one of the first and second stators. The stator camming elements match the camming members of the cam hub. The cam hub is rotatable in a first direction wherein the camming members of the cam hub urge the stator camming elements away from the rotor and a second direction wherein the camming members of the cam hub permit the stator camming elements to move closer to the rotor.
Preferably, each of the first and second mechanisms includes at least one resilient member arranged so as to urge a respective one of the first and second stators away from the rotor with a counter-active force which counteracts an attractive force between the respective stator and the rotor. The resilient member is arranged so that the counter-active force increases as the respective one of the first and second stators approaches a position associated with a minimum air gap. The resilient member therefore advantageously reduces the loading on thrust bearings of the motor.
Preferably, each of the first and second mechanisms further includes an additional resilient member for positively loading the respective one of the first and second stators toward the rotor so that the camming members remain in contact with the stator camming elements.
The present invention also provides a motor controller for controlling a multi-phase D.C. motor. The motor controller includes switching elements for connection electrically between a D.C. power supply and windings associated with respective phases of the motor, current detection lines which carry signals indicative of current flowing through respective ones of the windings, and a control unit responsive to the current detection lines.
The control unit controls each switching element to initially close at the beginning of a respective commutation step, then to open when current through a respective one of the windings achieves a first predetermined value and to close again when the current through the respective one of the windings drops below a second predetermined value. This process of opening (when the second predetermined value is reached) and closing (when the current falls to the second predetermined value) the switching elements continues over each commutation step.
The first and second predetermined values define a hysteresis bandwidth therebetween and are determined based on an intermediate value of current desired for the present commutation step of the motor. The control unit is arranged so as to selectively vary the hysteresis bandwidth.
The hysteresis bandwidth may be varied in a manner which minimizes the sum of switching power losses associated with the switching elements and resistive power losses associated with winding resistance. The hysteresis bandwidth also may be varied so that audible and/or undesirable noise is reduced.
The control unit may be responsive to a signal indicative of an amount of torque desired and may be arranged so as to set the first and second predetermined values accordingly.
In the context of a three-phase, wye-connected D.C. motor, the motor controller includes a first switch for connection electrically between a first terminal of a D.C. power supply and windings associated with a first phase of the motor, a second switch for connection electrically between the first terminal of the D.C. power supply and windings associated with a second phase of the motor, a third switch for connection electrically between the first terminal of the D.C. power supply and windings associated with a third phase of the motor, a fourth switch for connection electrically between a second terminal of the D.C. power supply and the windings associated with the first phase, a fifth switch for connection electrically between the second terminal of the D.C. power supply and the windings associated with the second phase, a sixth switch for connection electrically between the second terminal of the D.C. power supply and the windings associated with the third phase, current detection lines which carry signals indicative of current flowing through at least two of the windings, and a control unit responsive to the current detection lines.
The control unit controls each of the fourth, fifth and sixth switches so that during each commutation step of the motor a corresponding one of the fourth, fifth and sixth switches initially closes until current through a respective winding achieves a first predetermined value, at which time the corresponding one of the fourth, fifth and sixth switches opens and remains open until the current through the respective winding drops below a second predetermined value less than the first predetermined value, whereupon the corresponding one of the fourth, fifth, and sixth switches closes. This opening a closing process continues for each commutation step.
The first and second predetermined values define a hysteresis bandwidth therebetween and may be determined based on an intermediate value of current desired for a present commutation step of the motor. The control unit is arranged so as to close one of the first, second and third switches depending on the present commutation step of the motor.
In addition, the control unit is arranged so as to selectively vary the hysteresis bandwidth. The hysteresis bandwidth may be varied in any of the aforementioned ways.
Preferably, the motor controller further includes a current detection multiplexer connected between the current detection lines and the control unit. The current detection multiplexer is arranged so that, depending on which commutation step is present, the control unit receives from the current detection multiplexer one of: 1) a first signal indicative of a detected magnitude of current flowing through a first of the windings, 2) a second signal indicative of a detected magnitude of current flowing through a second of the windings, and 3) a third signal derived from the first and second signals and indicative of a magnitude of current flowing through a third of the windings.
The control unit preferably includes a programmable microcontroller capable of providing a commutation signal indicative of the present commutation step, a signal indicative of the first predetermined value, and a signal indicative of the second predetermined value. The commutation signal may be applied to the current detection multiplexer as a control signal.
The present invention also includes a current detector for providing outputs indicative of current flowing through each of three phase windings in a motor, based on detected magnitudes of current flowing through only two of the phase windings.
The current detector of the present invention includes a current detection multiplexer for connection to a signal indicative of a present commutation step of the motor. The current detection multiplexer is arranged so that, depending on which commutation step is present, the current detection multiplexer outputs one of 1) a first signal indicative of a detected magnitude of current flowing through a first of the windings, 2) a second signal indicative of a detected magnitude of current flowing through a second of the windings, and 3) a third signal derived from the first and second signals and indicative of a magnitude of current flowing through a third of the windings.
The current detector preferably includes a first current sensor for detecting the magnitude of current flowing through the first of the windings, and a second current sensor for detecting the magnitude of current flowing through the second of the windings. The current detector also may include a resistor network for scaling inputs to the current detection multiplexer.
According to yet another aspect of the present invention, a current detector for a three-phase motor includes a first input port for connection to a first current sensing device which senses current in a first winding of the three-phase motor, a second input port for connection to a second current sensing device which senses current in a second winding of the three-phase motor, and summing circuitry connected to the first and second input ports. The summing circuitry provides a sum of the magnitudes of current in the first and second windings of the three-phase motor.
The current detector further includes a switching mechanism for outputting during a commutation step:
1. a signal indicative of the current in the first winding if the current in the first winding was turned on at a beginning of a prior commutation step;
2. a signal indicative of the current in the second winding if the current in the second winding was turned on at the beginning of the prior commutation step, and
3. a signal indicative of the sum if current in a third one of the windings was turned on at the beginning of the prior commutation step.
The present invention also provides a discharge device for automatically discharging a residual voltage on a power bus after the residual voltage drops below a predetermined value. The discharge device includes control circuitry connectable to the power bus. The control circuitry is capable of determining whether the residual voltage has dropped below the predetermined value and is also capable of providing a control signal indicative thereof.
The discharge device further includes a discharge switching element responsive to the control signal from the control circuitry. The discharge switching element is arranged so that when the control signal indicates that the residual voltage has dropped below the predetermined value, the discharge switching element effects discharging of the residual voltage.
The above and other objects and advantages will become more readily apparent when reference is made to the following description taken in conjunction with the accompanying drawings.